1. Field of the Invention
The invention relates generally to processing integrated circuits and more specifically to lithography.
2. Discussion of the Related Art
Fabrication techniques for integrated circuits often rely on a patterned photoresist layer to protect underlying structures during etches and material depositions, i.e., depositions of metals, dielectrics, or semiconductors. Typically, forming a patterned photoresist layer is a process with multiple steps. The steps include depositing a photoresist layer on a substrate, exposing the photoresist layer to a selected light pattern, curing the photoresist to cause chemical changes therein, and developing the cured photoresist to dissolve away either the exposed regions or unexposed regions. The developing step produces a layer covered by a pattern of holes that is either a negative or a positive image of the light exposure pattern.
In the fabrication of a patterned photoresist layer, a shadow, proximity, or projection lithography technique typically produces the light pattern used for the exposure step. Forming the desired light pattern involves passing collimated source light through a mask. The mask has a pattern of opaque regions that blocks portions of the wavefront of the collimated source light. Thus, passing the source light through the mask produce a spatial light pattern for exposing the layer of photoresist.
In the integrated circuit (IC) industry, the masks for lithographic processes are often expensive to fabricate due to the need for complex and fine patterns of opaque regions on such masks. Furthermore, ICs often have a sequence of different vertical levels and the production of each level often requires its own mask pattern. That is, a different mask is often needed to form each of the different layers of an IC. For these reasons, mask costs can be a significant portion of the total cost for fabricating multi-layer electronic ICs.